Electrical, Optical and Structural Properties Of Functional Multilayers On Steel Sheets For Potential Applications In Cu(In,Ga)Se2 Solar Cells

  • Dodji Amouzou

    Student thesis: Doc typesDoctor of Sciences

    Abstract

    Cu(In,Ga)Se2, in short CIGS, solar cells have received considerable attention among photovoltaic solar cells because of their high conversion efficiency, flexibility and lightweight. The challenge in CIGS technology nowadays is not only to enhance performances but also to find low cost processing options. The use of CIGS modules built from monolithic interconnected cells on steel sheets is a promising solution; however it requires an additional dielectric layer interposed between the substrate and the entire active stack. Thus, the focus of this thesis was to elaborate a novel dielectric systems prepared on steel substrates and to optimize molybdenum layers as contacts for the fabrication of “high temperature” flexible CIGS solar cells. Polysilazane hybrid polymers were identified and explored to act as a dielectric layer for CIGS solar cells. Coatings were performed on stainless steel substrates using a straightforward “bar coating” deposition method which is suitable for industrial rolling processes. The problem with polysilazane coatings is that they often suffer from thermal instability and cracking due to the high annealing temperature required for growing the CIGS absorbing layer. Indeed, our thermal analysis reveals an important mass loss of around 25 wt. % for type of polysilazane we use. The mass reduction was assigned to chemical transformations of the polymer evidenced by volatilization of different gases and evolution of solvent. Such degradations show a severe weakening of the dielectric strength of polysilazane coatings. It was shown that polysilazanes can be stabilized after annealing at 550°C. Polysilazane single layers with a thickness below 2 µm are crack-free even after annealing but, unfortunately do not fulfill the dielectric requirements. Single layers with a thickness higher than 2 µm systematically cracked after annealing. With a subtle combination of three layers of polysilazane and a sputtered SiOxAly layer we succeeded in growing a crack-free dielectric stack with total thickness of more or less 4 µm. Constant voltage stress measurements performed on metal-insulating-metal capacitors reveal that such dielectric stacks can tolerate an applied voltage of 1000 V before annealing and 500 V after annealing at 500°C. Optimization of Mo films was then performed on dielectric stacks and a method to produce crack-free and well-adhered Mo films was established. Two solutions were found and each solution was obtained by combining two different layers. Both solutions pass adhesion testing and exhibit very low sheet resistance required for Mo back contacts. The interface between a set of Mo films and polysilazane was also studied. There seems to be a link between Mo-O-Si covalent bonds and good adhesion at the interface. Adhesion loss observed for some Mo/polysilazane samples occurred due to decohesion in polysilazane itself. The decohesion is assigned to the weakening of polysilazane due to the formation of molybdenum carbide at the interface. Finally, the analysis of steel modified by ultra-thin polysilazane reveals the presence of Cr-O-Si and Fe-O-Si covalent bonds which may explain the good adhesion observed with polysilazane/steel samples
    Date of Award31 Jan 2014
    Original languageEnglish
    Awarding Institution
    • University of Namur
    SupervisorROBERT SPORKEN (Supervisor), Guy Terwagne (Jury), Laurent HOUSSIAU (President), Jean-Pierre Raskin (Jury), Lionel Fourdrinier (Jury) & Sivalingam Sivananthan (Jury)

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